1. Field of the Invention
The present invention relates to a stage mechanism for fixing a stage, an electron microscope having the stage mechanism and a method of controlling positioning of the stage mechanism.
2. Background Art
A scanning electron microscope (SEM) is an apparatus for obtaining an SEM image of a specimen by scanning a specimen surface with a probing converged electron beam and detecting backscattered electrons and secondary electrons emitted from the specimen. Such microscopes using as a probe a charged particle beam such as an electron beam have been used in various fields of research and development and have also find applications in manufacturing fields. For example, a technique to obtain an SEM image of an object to be observed (specimen) with a scanning electron microscope is applied to observation and size measurement on semiconductor fine structures and is indispensable to semiconductor manufacturing processes.
In recent years, design rules for semiconductor integrated circuits have become finer. The pattern width is, for example, 100 nm or less in quite a number of cases. Semiconductor inspection/measurement apparatuses for performing size measurement and shape observation on fine patterns, detection of foreign materials and defects, etc., include critical dimension SEMs, SEM-type defect inspection apparatuses and review SEMs.
There is a demand for obtaining images with high accuracy as well as at a high throughput for a semiconductor inspection/measurement apparatus. Therefore, high-accuracy positioning by moving at a high speed and suppression of vibration and drift at the time of stopping are required of a specimen stage on which a wafer is mounted.
Structures using a feed screw as a drive means have been used for specimen stages in many cases. In recent years, stages having an ultrasonic motor, which is a linear actuator using a piezoelectric phenomenon of a piezoelectric ceramic element as a drive source have been proposed, for example, for the purpose of improving the positioning accuracy and avoiding a chemical contamination to a wafer derived from lubricating oil. An example of such stages is a “stage for electron microscope” disclosed in JP Patent Publication (Kokai) No. 3-129653 A (1991).
As a technique to provide a fixing mechanism for the purpose of preventing a wobble or small vibration of a stage after positioning, i.e., stage drift, a “target moving device” disclosed in JP Patent Publication (Kokai) No. 2001-196021 A (particularly in paragraph [0031], FIG. 3) is known. In this example, a structure is used in which a positive voltage is applied to a piezoelectric actuator provided on a stage to cause such a deformation extending to the left and right that braking pads are displaced in such a direction as to pinch a braking rail while being fulcrumed on a boundary portion between two column portions.